We identified a novel short (s-) mRNA of the human leukotriene-A4 (LTA4) hydrolase using sequential reverse transcriptase PCR mapping. The s-mRNA is generated by skipping an 83 bp exon located in the 3' coding region and suggests the expression of an LTA4 hydrolase isoform with a calculated molecular mass of 59 kDa and a distinct C-terminus. Both LTA4 hydrolase mRNAs are constitutively expressed in blood cells, endothelial cells, smooth muscle cells, fibroblasts and tumour cells. The ratios of their mRNA expression levels are cell specific, with relatively high s-form mRNA expression in reticulocytes. Our data strongly suggest that the novel mRNA codes for the structurally related but distinct LTA4 hydrolase isoenzyme that has been postulated.